WARNING: The following contains spoilers for Season 8 of Game of Thrones.
Ever since the series premiere, Game of Thrones has built up the Night King and his fellow White Walkers as an ultimate threat. These ancient undead creatures have plotted to invade Westeros and eradicate the living from the Seven Kingdoms as they bring an eternal winter with them wherever they go. This rampage culminated in the Night King's Army of the Dead attacking Winterfell in a coordinated assault to kill Bran Stark, the last Three-Eyed Raven, and the memory of humanity with him.
As the battle approached its bloody climax and the last of its defenders began to be overwhelmed by the undead, the Night King himself entered the destroyed castle to personally execute Bran who sat by the Godswood inside the castle walls, defended by Theon Greyjoy and his men, vowing to fight to the last in defense of the Three-Eyed Raven against the coming evil. It is there where one of the biggest twists of the entire series occurs in the final moments of the episode.
After easily killing Theon and his men, the Night King prepares to finish off Bran only for Arya Stark, who had been hiding in the Godswood's branches above, to leap in an ambush against the White Walker threatening her brother. Catching Arya by her neck with a single hand before she can deliver a killing blow, the Night King is caught off guard when Arya drops her Valyrian steel blade to her offhand and stabs him directly in the heart.
The White Walkers had been revealed to have a vulnerability to Valyrian steel, the metal used by the Children of the Forest to create the Night King in the Godswood centuries ago. The mythical beings stabbed a man bound to the tree in the heart with a Valryian steel blade to transform it into a weapon for the Children of the Forest to use in their war against humanity. This plan would ultimately backfire, forcing the Children of the Forest to team up with the First Men to drive back the White Walkers before their resurgence at the start of the series.
Stabbed in the heart by the very metal and next to the tree where he had been transformed in the first place, the Night King is immediately unmade, bursting into ice fragments along with his fellow White Walkers. With the figure that raised them from the grave no destroyed, the Army of the Dead and undead dragon Viserion similarly fall from their unlife; no longer threatening the world of the living.
Earlier in the episode, Melisandre had revealed that Beric Dondarrion, the holy knight in the Brotherhood without Banners, was prophesied to play a special role and unable to die until his destiny had been fulfilled. The Battle of Winterfell saw Beric deliver on this foretold promise by saving Arya at the expense of his own life from the undead that were hunting her within the castle walls. Arya's importance was then revealed when she personally killed the Night King once and for all, saving Westeros from the resurgent White Walkers.
It had been assumed by many that the final season of Game of Thrones would focus primarily on the fight against the White Walkers with the Seven Kingdoms staging a desperate defense against the undead threatening to eradicate the living.
With the Night King and White Walkers now evidently destroyed by Arya's epic last-ditch assault, it appears this won't be the case at all with Cersei Lannister preparing to destroy the defenders of Winterfell instead. This surprise defeat of the White Walkers, halfway through the season, is all thanks to a surprise attack from Arya, a character who has opposed death the most ever since the start of the series. Winter has come and gone thanks to Arya, but the war for the Iron Throne remains.
Airing Sundays at 9 p.m. ET on HBO, Game of Thrones stars Peter Dinklage as Tyrion Lannister, Nikolaj Coster-Waldau as Jaime Lannister, Lena Headey as Cersei Lannister, Emilia Clarke as Daenerys Targaryen, Sophie Turner as Sansa Stark, Maisie Williams as Arya Stark and Kit Harington as Jon Snow.